1. Field of the Invention
The present invention relates in general to a full-field three-dimensional imaging apparatus and method using a compressive sensing (CS) based tomographic imaging camera (TomICam) in conjunction with a swept-frequency laser source and readily available low-speed detector arrays such as CCD or CMOS cameras. Compressive sensing is employed for the 3-D imaging of targets that are known to comprise a small number of scatterers in the axial (z) direction, i.e., sparse axial scatterers. The CS-TomICam can drastically reduce the number of measurements necessary to generate a full 3-D image, leading to additional advantages such as lower power requirements and image acquisition speeds.
2. Description of the Background Art
Frequency modulated continuous wave (FMCW) reflectometry has emerged as a very important technique in a variety of applications including LIDAR [1], biomedical imaging [2, 3], biometrics [4], and non-contact profilometry [5]. This is due to unique advantages of the FMCW approach such as a high dynamic range and simple data acquisition that does not require high-speed electronics [6]. The basic principle of FMCW LIDAR is as follows. The optical frequency of a single mode laser is varied linearly with time, with a slope ξ. The output of the laser impinges on a target and the reflected signal is mixed with a part of the laser output in a photodetector (PD). If the relative delay between the two light paths is τ, the PD output is a sinusoidal current with frequency ξτ. The distance to the target (or “range”) τ is determined by taking a Fourier transform of the detected photocurrent. Reflections from multiple targets at different depths result in separate frequencies in the photocurrent.
The important metrics of an FMCW system are the linearity of the swept source—a highly linear source eliminates the need for post-processing of acquired data—and the total chirp bandwidth B which determines the range resolution. A high-resolution FMCW LIDAR or imaging system has two important components: i) a broadband swept-frequency laser (SFL) for high axial resolution; and ii) a technique to translate the one-pixel measurement laterally in two dimensions to obtain a full 3-D image.
State of the art SFL sources for biomedical and other imaging applications are typically mechanically tuned external cavity lasers where a rotating grating tunes the lasing frequency [2, 7, 8]. Fourier-domain mode locking [9] and quasi-phase continuous tuning [10] have been developed to further improve the tuning speed and lasing properties of these sources. However, all these approaches suffer from complex mechanical embodiments that limit their speed, linearity, coherence, size, reliability and ease of use and manufacture.
Detectors for 3-D imaging typically rely on the scanning of a single pixel measurement across the target to be imaged [11]. This approach requires a complex system of mechanical scanning elements to precisely move the optical beam from pixel to pixel, which severely limits the speed of image acquisition. It is therefore desirable to eliminate the requirement for mechanical scanning, and obtain the information from the entire field of pixels in one shot. To extend the FMCW technique to a detector array, the frequencies of the photocurrents from each detector in the array should be separately calculated. However, in a high-axial-resolution system, each detector in the array measures a beat signal typically in the MHz regime. A large array of high speed detectors therefore needs to operate at impractical data rates (˜THz) and is prohibitively expensive. For this reason, there are no practical full-field FMCW LIDAR imaging systems, except some demonstrations with extremely slow scanning rates [4, 11] or expensive small arrays [12].
An ideal FMCW LIDAR system will therefore consist of a broadband rapidly tuned SFL, and a detection technique that is capable of measuring the lateral extent of the object in one shot. The system will be inexpensive, robust, and contain no moving parts.
Previously, a novel optoelectronic SFL source has been developed [13] based on the tuning of the frequency of a semiconductor laser via its injection current. Using a combination of open loop predistortion and closed loop feedback control of the laser current, the SFL generates extremely linear and broadband optical chirps. The starting frequency and slope of the optical chirp are locked to, and determined solely by, an electronic reference oscillator—they are independent of the tuning characteristics of the laser. Chirp bandwidths of 1 THz at chirp speeds exceeding 1016 Hz/s have been demonstrated, and it has been shown that arbitrary optical chirp shapes can be electronically generated. The optoelectronic SFL source is compact and robust, has low phase noise and large chirp bandwidth, and has no moving parts.
The invention disclosed in the '962 application provides a detection approach for FMCW LIDAR, in which the frequencies of the signals employed by the apparatus are modified in such a manner that low-cost and low-speed photodetector arrays, such as CCD or CMOS cameras, can be employed in a tomographic imaging camera (TomICam). The approach obviates the need for high-speed detector arrays for full-field imaging, and thus leads to a practical approach to measure FMCW LIDAR signals on an array of pixels in parallel.
In the operation of the TomICam, by first modulating or translating the frequency of at least one of the target or reference beams, the difference between the frequencies of the reflected and reference beams is reduced to a level that is within the bandwidth of the detector array. Thus, the need for high-speed detector arrays for full-field imaging is obviated. The key insight is thus that the measurement of the photocurrent frequency, which determines the distance to the illuminated object or target imaged by a detector array pixel, can be moved to a lower frequency by modulating the optical frequency of at least one arm of the interferometer (e.g., the reference arm or the “Local Oscillator” (LO) arm) using an optical frequency shifter, for example. By using a low-speed photodetector, which effectively acts as a low pass DC filter, all components other than the DC term are filtered out, leaving only the detected value which is proportional to the square root of the reflectivity of the target at the selected range.
Thus, a single pixel measurement using the TomICam yields the value of any target reflections present at a particular distance. The array of low-speed photodetectors can therefore be used to image a lateral two-dimensional “tomographic slice.” In the case of a frequency shifter, by electronically varying the value of the frequency shift, tomographic slices at different depths can be obtained and combined to form a full three-dimensional image. Thus N measurements are necessary to measure N possible target depths, where N is determined from the resolution and swept frequency bandwidth of the SFL.
This highlights one potential tradeoff to using the TomICam approach, which is that regardless of the number of targets to be detected, N measurements must still be made to obtain a full 3-D image. In the case of a small number of targets, which is many times the case in typical FMCW LIDAR applications, this process becomes inefficient.